Rigid Swimmer and the Cretaceous Ichthyosaur Revolution (part I)

You’re reading a blog. This almost guarantees the fact that you’re a staunch supporter, and fan, of open-access publishing. Many of us who publish technical research really do try to publish in open-access venues as often as possible. Doing so is generally good for citation indices and all that, but it’s especially important if you care about outreach and about the public availability of your research. I’ve published in open-access journals before (Witton & Naish 2008, Taylor et al. 2009), and I’m very happy to say that my very newest paper – it’s out today – is published in the open-access, 100% available-to-all journal PLoS ONE.

The article concerned is co-authored with Valentin Fischer, Michael Maisch and others and is predominantly based around the naming of a very interesting new Cretaceous ichthyosaur (Fischer et al. 2012). As we explain – and as I’ll summarise here – it isn’t just any old ichthyosaur; it has major ramifications for our understanding of ichthyosaur diversity across time, and especially about their history across the Jurassic-Cretaceous boundary (known for short as the J/K boundary event or as the JCB event).

For some decades now the general feeling about ichthyosaur history and diversity has been that things were going very well for ichthyosaurs during the Late Triassic and Early Jurassic, that diversity declined during the Middle and Late Jurassic, and that ichthyosaur diversity during the Cretaceous was so low that the group was essentially just lingering on and doomed never to do anything interesting ever again. The JCB was regarded as a major extinction event for ichthyosaurs, with the successful and globally distributed ophthalmosaurine thunnosaurians being killed off, their place taken by the supposedly “much more generalized” platypterygiines (Bakker 1993). This fits with wider evidence showing that other marine groups (including thalattosuchian crocodilians and plesiosaurs) were affected quite seriously by the JCB event (Benson et al. 2010).

Skull of the platypterygiine Brachypterygius, from McGowan & Motani (2003). This skull was originally used to erect the new taxon Grendelius mordax - an awesome name. Grendelius is certainly synonymous with Brachypterygius, but opinions differ as to whether B. mordax is distinct from the type species, B. extremus.

Of ophthalmosaurines and platypterygiines

For ease of communication, I have to explain here that I and my co-authors follow a system where ophthalmosaurines and platypterygiines are grouped together into the more inclusive clade Ophthalmosauridae. Ophthalmosauridae is part of Thunnosauria, an ichthyosaurian clade that also includes Ichthyosaurus and Stenopterygius.

Many ichthyosaur workers have long suspected a divergence of Ophthalmosauridae into distinct ophthalmosaurine and platypterygiine clades, so one of the most satisfying phylogenetic aspects of our study is that we were able to find robust support for this division (Fischer et al. 2012). Because there are ophthalmosaurines from the Middle Jurassic (like Mollesaurus), the ophthalmosaurine/platypterygiine divergence must have occurred by this time.

Ophthalmosaurus icenicus, best known ophthalmosaurine. The life restoration is by Ken Kirkland; the skeletal is from Sander (2000).

Ophthalmosaurines were big-eyed, deep-diving thunnosaurians, well known for possessing a tuna-like body shape and hence likely sharing with tuna (and other thunniform vertebrates) a high swimming speed and endothermic physiology (Motani 2002, 2005). Platypterygiines have proportionally smaller eyes, and also have a more robust snout form, larger teeth, and a longer, less tuna-like body shape. If the JCB was a time of ecological turmoil and reorganisation (as often seems to be the case during extinction events), it would seem logical that ophthalmosaurines – conceivably tied to specific and reliable resources – would become extinct, whereas generalist platypterygiines – able to make a living whatever the available prey – would survive (this view of platypterygiines as generalists is supported by data from stomach contents: Kear et al. 2003). This tidy story of major ichthyosaur turnover at the JCB was stated most specifically and confidently by Bakker (1993). There is a problem though: it’s wrong.

Secondly, a list of taxa originally described from the Upper Jurassic – namely Brachypterygius, Aegirosaurus, Caypullisaurus and Yasykovia – have now been reported from Lower Cretaceous sediments (McGowan & Motani 2003, Fernández & Aguirre-Urreta 2005, Efimov & Komarov 2010, Fischer et al. 2011). Additionally, Ophthalmosaurus – originally described from the Middle Jurassic – has been reported from the Lower Cretaceous of Canada and England (McGowan & Motani 2003). These Cretaceous Ophthalmosaurus records are ambiguous given that the characters used to refer them to Ophthalmosaurus are now known to have a broader distribution. Additional English records, however, can at least be identified as cf. Ophthalmosaurus (Fischer et al. 2011). The presence of these many ‘Jurassic’ ophthalmosaurids in the Cretaceous shows that the JCB was not a major extinction event for ichthyosaurs – many lineages crossed the boundary just fine (Fischer et al. 2011).

Neoichthyosaurian phylogeny plotted against time, from Fischer et al. (2012). Note how many ophthalmosaurid lineages cross the JCB.

Thirdly, the ichthyosaur taxon that (prior to these new discoveries) was typically thought to be the only Cretaceous representative of the group – globally distributed, super-speciose Platypterygius – is increasingly being regarded as a ‘taxonomic wastebasket’ that actually contains a higher diversity than previously supposed (e.g., Maxwell & Caldwell 2006, Fischer in press). In other words, the many species currently included within Platypterygius are almost certainly not all close relatives, and Platypterygius is (I predict) eventually going to be found non-monophyletic. The outcome of such a conclusion will be its dissolution into several discrete ‘genera’ (several authors have already gone down this route, but their efforts to name new genera for the platypterygiine taxa concerned haven’t been widely accepted).

Yet another new Cretaceous ichthyosaur

GLAHM 132588, the Acamptonectes densus holotype.

Our new paper adds to this story, for we describe another new Cretaceous ichthyosaur. Yes, say hello to Acamptonectes densus Fischer et al., 2012 from the Lower Cretaceous of England and Germany. The history of our research on Acamptonectes is kind of interesting. This is one of several new ichthyosaur taxa originally studied by the late Robert Appleby; the best specimen [shown above] is from the Speeton Clay of North Yorkshire in England, where it was discovered in 1958. Despite penning an essentially complete description, Appleby never got round to publishing it. Within recent years, I worked with Jeff Liston and the late Arthur Cruickshank to get several of Appleby’s unpublished manuscripts into print. However, Appleby regarded Acamptonectes as a new species of Platypterygius (dubbed ‘P. speetoni’). This opened up a whole world of hurt, since I couldn’t finish the project without comparing the Speeton Clay ichthyosaur to the many new platypterygiine taxa named since Appleby had stopped his work on this taxon.

Skull (and part of neck) of the Crelimgen Acamptonectes, from Fischer et al. (2012).

Unbeknownst to me, a new Lower Cretaceous ophthalmosaurid ichthyosaur had been discovered in Cremlingen, Germany, in 2003. It was being studied by Valentin Fischer, Michael Maisch and a number of collaborators. It represented a new taxon, but the characters that made it distinctive and unique compared to other ophthalmosaurids were also present in the Speeton Clay animal. In other words, they represented the same new animal. So, when Valentin visited the Speeton Clay ichthyosaur at its home in Glasgow’s Hunterian Museum in Scotland (Jeff’s former academic base), collaboration was born. We ended up using the Speeton Clay specimen as the holotype and the Cremlington specimen (a subadult) as a paratype (Fischer et al. 2012). A second Speeton Clay specimen, discovered in 1985, is in the collections of the Natural History Museum in London and was used by us as a second paratype. Both the Speeton Clay and the Cremlingen strata that yielded the Acamptonectes specimens are Hauterivian in age, but the former are early Hauterivian and the latter are late Hauterivian (Fischer et al. 2012 and references within).

Reconstructed skeleton of Ophthalmosaurus icenicus from the English Oxford Clay, by Captmondo.

It was always obvious that the new animal was an ophthalmosaurid, as it shares a large number of characters with Ophthalmosaurus, Brachypterygius, Mollesaurus and their relatives (distinctive ophthalmosaurid characters include the form of the basioccipital and the presence of three facets on the distal end of the humerus). Within Ophthalmosauridae, Acamptonectes lacks the numerous derived characters that unite the members of Platypterygiinae, and in a phylogenetic analysis it grouped close to Ophthalmosaurus (Fischer et al. 2012). Appleby was wrong in thinking that Acamptonectes was close to (or part of) Platypterygius; instead, it’s an ophthalmosaurine. [Ophthalmosaurus photo by Captmondo, from wikipedia.]

The ophthalmosaurines we know of are conservative, and approximately similar in body shape and proportions. So, what makes Acamptonectes so special? Its snout is shallow compared to that of its relatives, and its teeth are slender-crowned and with sharply pointed crowns (suggestions that some ophthalmosaurines were toothless are not correct). Compared to other ophthalmosaurines, it has features indicating that parts of its skeleton were unusually rigid and firmly locked together. The head of the slender-shafted stapes is expanded and massive and, in life, would have had a particularly strong union with the adjacent opisthotic and basioccipital bones. The basioccipital is typically a very important bone in ichthyosaur identification, and that of Acamptonectes is no exception, possessing an unusual bilobed concavity on its dorsal surface.

The centra of the cervical and dorsal vertebrae have peculiar, S-shaped articular surfaces, and they interlock tightly. This means that the anterior part of the vertebral column was very stiff, with hardly any side-to-side motion being possible. This might sound odd for an animal that swims with lateral movements of its vertical tail fin, but it’s an accentuated version of a trend seen in other thunnosaurian ichthyosaurs. As they came to rely more and more on the distal, finned part of the tail for propulsion (rather than on the tail as a whole), they stiffened up the body. Similar trends are seen in other vertebrates that have become specialised for so-called carangiform swimming: in lamnid sharks, tunas and billfishes, for example, there isn’t much lateral movement in the body or anterior part of the tail either.

Another distinctive feature of Acamptonectes is that its ribs are rounded in cross-section. In other thunnosaurians, deep grooves on both surfaces of the rib shaft mean that each rib is 8-shaped in cross-section. The rounded and rather robust rib shafts of Acamptonectes were presumably more resistant to bending than those of other thunnosaurians – is this a further adaptation to increase stiffness in the animal’s body?

Combined, these unusual features of the skull and axial skeleton in particular inspired us to come up with the name Acamptonectes densus Fischer et al., 2012 for our new animal. It means something like ‘tightly packed rigid swimmer’, and it refers to the chunky, tightly fitting nature of the occipital bones and vertebrae.

While Acamptonectes was especially robust in some regards, it’s worth saying that the massive, stout proportions of ophthalmosaurid anatomy are not sufficiently appreciated. You might get the impression from artistic reconstructions that ophthalmosaurids (both ophthalmosaurines and platypterygiines) were similar in overall proportions to, say, bottlenose dolphins, and with laterally compressed fore- and hindpaddles. In fact, their skulls are often extremely broad and chunky across the occipital region and the rostrum is usually very broad-based and deep at the level of the nostril. Ophthalmosaurid forepaddles are generally enormous: they’re very broad (and long in platypterygiines), but also surprisingly thick, with very prominent bony crests on the humerus meaning that the paddle was massively muscled [check out the impressive thickness on the humerus and phalanges in the ophthalmosaurid forefin shown in the adjacent photo]. Even the oval or rectangular phalanges in the more distal parts of the paddle are thick and robust, and not delicate tiles as you might assume.

Acamptonectes to the end

Our holotype and paratype specimens of Acamptonectes are from the Hauterivian. But it gets better: numerous fossils referable to Acamptonectes are also present in the English Cambridge Greensand. This is lower Cenomanian in age and thus about 30 million years younger than the late Hauterivian strata that yielded the Cremlington A. densus specimen. While the Cambridge Greensand fossils can be confidently referred to Acamptonectes, they differ from the A. densus specimens in various details (in possessing, for example, a larger extracondylar area on the basioccipital). This means that they can be identified as Acamptonectes sp., but not necessarily as A. densus (Fischer et al. 2012) – a conclusion that seems reasonable in any case given that c. 30 million year difference in age. More material is needed before we can determine with precision the specific status of the Cambridge Greensand Acamptonectes. Anyway, the most interesting thing here is that Acamptonectes was present from the early Hauterivian until the early Cenomanian.

The basisphenoid (a bone from the very bottom of the braincase) described as Ichthyosaurus brunsvicensis by Broili (1909). L: ventral view. R: anterior view.

Of further interest here is that a very Acamptonectes-like ophthalmosaurid was described from the Cretaceous of Germany in 1909 and named Ichthyosaurus brunsvicensis. This name has generally been regarded as a nomen dubium, and the specimen lacked the diagnostic characters of Acamptonectes (like that bilobed cavity on the basioccipital). The situation isn’t helped by the fact that the specimen was destroyed during WWII. We identify the 1909 specimen as cf. Acamptonectes and hope that new material will one day be found that allows it to be confirmed as an additional ophthalmosaurine taxon.

Anyway, we can now say that Acamptonectes was present until the Cenomanian. This is particularly interesting in the context of Cretaceous ichthyosaur diversity since it shows that Ophthalmosaurinae was long-lived, and that it persisted until virtually the end of recorded ichthyosaur history. Previously, only platypterygiines were known from these younger Cretaceous strata. The last ichthyosaurs, then, were not lone representatives of a single lineage: rather, “the ‘last’ ichthyosaurs were actually taxonomically diverse and morphologically disparate, making their Cenomanian extinction far more severe than previously assumed” (Fischer et al. 2012, p. 20).

Ichthyosaur diversity plotted against time, from Fischer et al. (2011). The dark parts of the graph show how the number of known taxa has changed since 2003. It's obvious that Cretaceous ichthyosaur diversity is now rather higher than previously thought.

New evidence, then, does not support the idea that the JCB event was a major event in ichthyosaur evolution as previously thought. Rather, ichthyosaurs of numerous lineages apparently sailed across the boundary without event. Quite why this is so when there’s good evidence that other marine reptile groups were affected quite severely by the JCB event is now a very good question – were ichthyosaurs actually highly resistant to whatever events caused this biological crisis? Furthermore, Early Cretaceous ichthyosaurs were not rare, sorry stragglers of a previous Golden Age – we are learning that ophthalmosaurids, at least, were doing quite well at this time, with diversity and disparity being reasonably high. Finally, ophthalmosaurines and platypterygiines both made it to the Cenomanian, suggesting that both clades were similarly affected by the same Cenomanian extinction event. Gradually, we are revising our views of what marine reptile diversity was like in the Cretaceous.

And, as I so often say, there is more to come.

UPDATE: A far more detailed, more historically accurate account of the story behind the work on the Speeton Clay 1958 A. densus specimen has been written up by Jeff Liston for Mr Wood’s Fossils.

UPDATE: while we’re here… some of you will be aware of the Research Works Act, a vile, nefarious attempt by some academic publishers to RESTRICT ACCESS TO ACADEMIC PUBLICATIONS AS MUCH AS POSSIBLE. We need to act (and act before January 12th) – please read the article here.

Efimov, D. V. & Komarov, V. N. 2010. The first find of fragments of a skeleton of the ichthyosaur Yasykovia in the Valanginian of Crimea. In Baraboshkin, E., Blagoveschensky, I. V. (eds) Cretaceous System of Russia and CISStates: Problems of Stratigraphy and Palaeogeography Materials of the 5th All-Russian Meeting (August 23-28, 2010, Ulianovsk), Ulyanovsk, pp. 132–135.

Fischer, V. In press. New data on the ichthyosaur Platypterygius hercynicus and its implications for the validity of the genus. Acta Palaeontologica Polonica.

Fischer, V., Clément, A., Guiomar, M., & Godefroit, P. (2011). The first definite record of a Valanginian ichthyosaur and its implication for the evolution of post-Liassic Ichthyosauria. Cretaceous Research, 32, 155-163

Maxwell, E. E. & Caldwell, M. W. 2006. Evidence for a second species of the ichthyosaur Platypterygius in North America: a new record from the Loon River Formation (Lower Cretaceous) of northwestern Canada. Canadian Journal of Earth Sciences 43, 1291-1295.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Darren Naish

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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